The mammalian biotransformation of the anthracycline glycosides and the immunosuppressive oxisuran illustrate two unappreciated phenomena in mammalian biochemstry. The microsomal reductive glycosidase is an enzyme which is controlled by the oxygen content of the tissue. Such enzymes are likely important under hypoxic conditions. The aldo-keto reductases exemplified by rat liver aldehyde reductase and oxisuran reductase are drug metabolizing enzymes of the cellular cytoplasm. These reductases have very similar characteristics and may be related.